Ultrasound has been established in the field of medicine for many years, mainly used as an imaging method to help monitor the status of a woman's fetus. The term ultrasound can be defined as sound with a frequency higher than that perceivable by the human ear (a range of roughly 20 Hz to 20 kHz). Medical ultrasound imaging and associated research typically takes place in the 1 MHz to 10 MHz range.
The first application of this technique in the field of medicine can be attributed to Dr. Karl Theodore Dussik. In 1952, Austria, he pioneered the field of medical ultrasonics, recounting his work done on transmission ultrasound through the brain, Professor Ian Donald further explored other applications of this technique in the late 50's and 60's. After extensive testing on abdominal masses, he conducted the very first trial of medical ultrasound on a pregnant woman in 1958.
Ultrasound techniques for medical applications have become popular due to the ease of use and non-invasive features. Ensuing years brought many improvements to the ultrasound probes, enabling higher resolution images.
When ultrasound strikes a surface object, some of it is reflected, scattered, or transmitted through the object, much like light passing through a lens. This sound is also attenuated when hitting the surface, with higher frequencies affected more than lower frequencies. Low frequency sounds can therefore traverse more layers of matter before being attenuated completely.
In medical ultrasonics, ultrasound is created by a transducer, a tiny piezoelectric device mounted inside a probe. When a current is run through this device, it vibrates at a specific frequency, generating ultrasound waves that emanate in the direction of the probe. The probe also doubles as an ultrasonic detector. When ultrasound hits the piezoelectric device, it vibrates and generates a current.
To ensure that high frequency ultrasonic waves propagate through tissue while minimizing attenuation due to striking a surface, clinical ultrasound probes need to be water-coupled to the tissue body being analyzed. This is achieved using an ultrasound gel, a substance rubbed onto the skin of a patient to provide full contact with the ultrasound probe.
An image can be generated from ultrasound by analyzing the reflections once it has propagated through layers of tissue. The time it takes for the reflections to return to the probe indicates the distance which the ultrasound pulse has traveled. Multiple layers of tissue can be perceived by scanning one spot of the body and listening to the multiple reflections returning to the probe. A complete image can also be generated by scanning a section of the body and aligning all the data from the ultrasonic reflections.
The integration of ultrasonic imaging in the field of medicine allowed a step by step approach to prenatal care in the womb. However, this type of imaging does have some drawbacks.
The first is the tradeoff between depth and resolution. As previously stated, lower frequency sounds (longer wavelength) travel deeper into objects, while higher frequencies (shorter wavelength) reveal very fine details, increasing imaging resolution.
To obtain the best possible resolution, it is preferable to uses a high frequency ultrasound. However, such high frequency ultrasound is attenuated very quickly and therefore does not penetrate very far into the human body. In order to traverse several levels of tissue and organs while still providing reasonable imaging capabilities, the frequency must be lowered, thereby sacrificing resolution.
The second major limitation relates to the lack of molecular modulation provided by medical ultrasound. Ultrasonic tissue imaging is very effective at illustrating the state of internal body parts as well as fetuses, however no modulation is gained by present methods of ultrasound with respect to the concentration of any specific molecules in the circulation or in tissues or organs. Other more invasive and often less desirable means are used when this modulation is required.
Therefore, there is a need for improved methods of detection using ultrasound devices.